This application is based on and claims priority from Japanese Application No. HEI 3-128915 filed Mar. 18, 1991, the disclosure of which is incorporated herein by reference.
The present invention relates to a zoom lens system having three lens groups that is primarily intended to be used with still cameras.
Conventional zoom lens systems include a fixed third lens group having a weak optical power, such, as disclosed in Unexamined Published Japanese Patent Application No. 120311/1985 (example 1) and No. 240217/1986 (example 2). An alternative conventional system moves the third lens group monotonically, such as disclosed in Unexamined Published Japanese Patent Application No. 26754/1979 (example 3).
However, in the zoom lens systems of examples 1 and 2, when the overall lens length is reduced during a zoom operation (i.e. when the system is between from a narrow-angle lens position to a wide-angle lens position) astigmatism varies. If these systems are set to provide zero astigmatism when the zoom is positioned at the narrow and wide-angle end positions, examples 1 and 2 experience negative astigmatism when the zoom moves between these end positions (i.e through the middle-angle position).
In each of the conventional zoom lens systems disclosed in examples 1 and 2 above, the second lens group must move a much larger distance than the first lens group when zooming to the narrow-angle end position. This motion differential excessively complicates the mechanism for performing focusing and zooming operations in a "one-hand" lens barrel, since both operations are performed with a single adjusting ring.
The zoom lens system of example 3, which moves the third lens group monotonically (i.e. in one direction as the zoom moves between narrow and wide angle end positions), requires a bulky overall lens system and a large number of lens to achieve a wide range of lens movement.